In a superconducting coil, when the temperature or the electric current or the like do not meet certain requirements to maintain the superconducting state, the so-called quench occurs and the superconducting state is shifted to the non-superconducting state. When the quench occurs, the resistance value is increased at the part where the quench occurs, and then, rapid conversion into the thermal energy occurs due to the electric current. In order to protect superconducting material and insulating material and prevent damages thereof when the quench occurs, earlier detection of the quench is desired.
In a direct current magnet that is formed by supplying a direct current in a winding of a superconductor, the quench is detected by a measurement value of the difference in the potential between two points of the winding. However, in the case of the SMES (Superconducting Magnetic Energy Storage), which stores electric power by means of a superconducting coil and discharge the electric power as demand, same quench detection as that in the case of the direct current magnet cannot be made since a high alternation voltage is applied to the SMES from the outside upon charging and discharging. In addition, the SMES is not capable of employing a protection circuit that is formed by placing a diode in parallel to the same winding as the direct current magnet. Therefore, it is necessary for the SMES to provide a dedicated protecting means thereto.
As an example of documents of conventional techniques with regard to the detection of the quench of a superconducting coil, Japanese Laid-Open Patent Application JP-P2001-274014A is referred to here.